Pressure sensitive adhesives are widely used in the adhesive industry. Large quantities of pressure sensitive adhesives (PSAs) are used in the production of masking tape and labels. Conventional PSAs may contain natural rubber or other similar elastomeric compounds and are either solvent-based or water-based. In solvent-based PSAs, the solids are dissolved in an organic liquid. In water-based PSAs, the solid components are dispersed in water.
In response to safety and environmental concerns, the use of water-based PSAs has grown. Though most water-based PSAs exhibit good shear strength, their solvent-based counterparts exhibit superior shear strength. In addition, natural rubber latex often exhibits poor mechanical stability and thus limits the methods by which it can be applied.
U.S. Pat. No. 4,011,388 describes a process for the preparation of aqueous homogeneous polymer emulsion which comprises mixing, generally with agitation or under shear conditions, water, surfactant, at least one polymer, e.g. cellulosic compounds, and at least one monomer, e.g. acrylics, methacrylics, styrene, etc., the polymer being soluble in said monomer, to form a dispersion of polymer-monomer particles in water; then subjecting said dispersion to conditions such that the monomer within the particles become polymerized by free radical polymerization thereby forming a substantially homogeneous blend of at least two polymers within the particles of the emulsion.
U.S. Pat. No. 5,686,518 describes a process for incorporating a polymer soluble in one or more of the monomers into an emulsion polymer. The monomer mix is subjected to high shear to form small droplets in the range of 10 to about 500 nm to form a mini-emulsion. The incorporated polymer level is limited to 0.5-5.0% based on the combined weight of monomer and polymer.
U.S. Pat. No. 5,071,904 describes a process for incorporating a hydrophobe (greater than 30 weight % based on combined weight of monomer and polymer) into a microparticle. Their system requires the use of a functional polymer (eg. hydroxyl group) and the use of a cross-linking agent (e.g. melamine formaldehyde or a blocked polyisocyanate).
U.S. Pat. No. 5,569,715 describes a process for forming a hydrophobically modified core polymer as part of a core-shell polymer. The core is made by mixing a hydrophobic polymer, monomer(s), water, and surfactant then subjecting to high shear to form surfactant stabilized particles (droplets) between 100 and 300 nm. This core material is polymerized. Then a hydrophilic shell is formed using conventional emulsion polymerization with at least one hydrophilic monomer.
WO 98/42761 describes a process for modifying natural rubber by swelling existing natural rubber latex particles with small quantities of vinyl and/or acrylic monomer, then initiating reaction of the monomer in the swollen latex particles. The process is most similar to a seeded emulsion polymer process.
U.S. Pat. No. 4,346,199 describes the use of myrcene to promote grafting of natural rubber latex to alkenyl aromatic and alkenyl nitrile monomers. The grafted and non-grafted portions of the final product are separated and made into sheeting.
U.S. Pat. No. 5,728,579 describes a method for blending various latexes, curing agents, tackifiers, and plasticizers followed by a heated curing step for use in pressure sensitive adhesive applications.
Japanese Patent 05287121 describes a process for modifying a natural rubber latex by adding small amounts of acrylic and vinyl monomers plus additional water and surfactant, then polymerizing the acrylic monomers through the addition of heat and an initiator. The acrylic monomers graft to the natural rubber forming an acrylic-modified natural rubber latex. However, these processes start with natural rubber in latex form and do not provide for pre-dissolution of rubber(s) into acrylic and/or vinyl monomer(s), forming of a mini-emulsion, and subsequent polymerization to form a hybrid latex.
U.S. Pat. No. 5,095,065 (Yang) describes a process for incorporating a low molecular weight tackifier by pre-dissolution in monomer followed by incorporation into a latex under conventional emulsion polymerization conditions.
U.S. Pat. No. 6,333,378 (Clark et al.) describes water-based latexes of a acrylic-modified alkyd or uralkyd resin or hybrid resin which can be prepared by emulsion polymerization as well as by mini-emulsion polymerization.
There exists a need in the art for water-based adhesives that afford both excellent shear strength and mechanical stability without the production of undesirable scrap levels. The invention, as described below, addresses such a need.